ACPO guideline is 10%+2 ie 35 in a 30, 46 in a 40, 57 in a 50 etc. etc. Most police forces adhere to this guideline as one of the reasons behind it is to prevent time wasted by people challenging the accuracy after getting done for 31 in a 30.
In any case the radar on a Gatso is NOT accurate and it will go off incorrectly from time to time, even if you are below the limit. Does that matter? Not in the slightest.
A Gatso takes two pictures 0.5 seconds apart and you'll notice you're driving along what is effectively a giant ruler, from this they know exactly what distance you have covered in those 0.5 seconds. As we learned in KS3 maths (well I did, who knows what they're teaching kids these days) speed = distance/time.
When the film is taken out of the camera they go over all the images and check the actual speed that way and if you're at 35 or over then an offence is entered into their system and an NIP is sent to you.
So, just because you have been flashed doesn't mean you're going to get a ticket.
Now, they do make mistakes from time to time. Not as many as the papers would have you believe but enough that it is a serious consideration. Most if not all camera partnerships will let you make an appointment to come in and view the photographs, if you genuinely believe that your speedo was only reading 33 then it's worth doing this, go in to view the photographs and ask them to go over the speed calculation with you. This will mean them doing the calculation again and doing it properly since they will be explaining it to you as they go along.
If the result is <= 30 then they'll apologise and cancel the offence there and then if it's between 31 and 34 then you might have to ask them about the ACPO guidelines and if they follow them.
In my experience the people who work at the camera partnerships are decent and honest people who genuinely believe that what they are doing is improving road safety and saving lives. I disagree with them on this point but the reason I say this is because if they do genuinely **** up they will be decent enough to cancel the offence and apologise. They're not a bunch of evil and scheming monsters trying to devise new ways to make driving more miserable and force people off the road...
...that's the government's job.